Servo control apparatus for optical disc player

ABSTRACT

A servo control apparatus of an optical pickup includes an extractor for extracting an error signal from a read signal; an equalizer including a phase compensator for equalizing the error signal to generate a drive signal; a driver for changing the servo position of the optical pickup in response to the drive signal; a dropout detector for extracting an envelope signal from the read signal to detect a dropout of the read signal; and a controller. The controller changes an equalization characteristics of the equalizer in accordance with an amplitude of the envelope signal during the period of time when the dropout occurs or in response to an occurrence of the dropout.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an optical pickup servo-controlapparatus for use with an optical disc player.

[0003] 2. Description of the Related Art

[0004] Servo control is usually used to control an optical pickup forreading recorded information at a desired reading position for thepurpose of achieving good reproduction performance in a player foroptical discs, such as compact discs (CD) and digital versatile discs(DVD). An optical disc, however, usually contains defects, such asvarious kinds of flaws and contamination which occur during, forexample, the manufacturing process or the use of the disc. Such defectsimpede a stable operation of the above-mentioned servo control.

[0005]FIG. 1 is a sectional view of an optical disc 1, showing the kindsof major defects which may occur. The optical disc 1 includes aprotection layer 3, a reflective layer 4 which reflects the light beamemitted from the optical pickup, and a transparent cover layer 5 made ofa transparent material such as a plastic and the like.

[0006] Referring to FIG. 1, a defect I is a flaw called an“interruption” in the reflective layer 4, caused in the manufacturingprocess of the disc. A defect II is a dirt called a “black dot” on thedisc's surface or the transparent cover layer 5. A defect III is a“fingerprint” of human fat on the disc surface 7. A defect IV is a flawcalled a “scratch” on the disc surface 7.

[0007] The existence of such defects on a disc causes “dropout” in readsignal (RF signal) generated in the optical pickup when the disc isplayed, and deteriorates the controllability of servo control includingtracking control, focusing control, tilt control and the like. Aconventional method, for example, for preventing the adverse influenceon servo control caused by such defect is to continue servo control byholding the control value of servo control, or the tracking error value,focus error value, or the like to the value before the defect wasdetected (hereinafter, simply referred to as “pre-value hold”).

[0008] The servo control, however, becomes unstable when the dropoutcontinues for a relatively long time or the dropout is of a burst type,because a bias or deviation value indicated by a value in thepre-value-hold method (hereinafter, simply referred to as a “holdvalue”) drifts too far from the actual bias value. For example, thedrive signal for the optical pickup drastically fluctuates during theperiod when a dropout occurs, causing servo control instability as shownin FIG. 2. In addition, as the transient phenomenon arises aftertermination of the dropout, servo control becomes unstable because thefocus error signal is delayed in converging into a normal value, forexample, as shown in FIG. 3. Therefore, the performance of servo controlmay be deteriorated or a gross deterioration in reproduction quality maybe caused.

[0009] The defect such as a fingerprint or a black dot is usually formedover a plurality of tracks in the radial direction of the disc (i.e., ina certain range of a rotational angle of the disc). Servo control alsoneeds to be stabilized effectively against such defects.

OBJECT AND SUMMARY OF THE INVENTION

[0010] Therefore, it is an object of the present invention to provide aservo control apparatus which is capable of performing stablehigh-precision servo control during the period of time when a dropoutoccurs and after the termination of the dropout even if the dropoutcontinues for a long time or the dropout is of a burst type.

[0011] It is another object of the present invention to provide a servocontrol apparatus which has a high capability of dealing with variouskinds of defects and of performing stable high-precision servo control.

[0012] To achieve the object, according to the present invention, thereis provided a servo control apparatus of an optical pickup for readingrecorded information from a recording medium to generate a read signal,which comprises an error signal extractor for extracting an error signalfrom the read signal, the error signal indicating a deviation from aservo target value of the optical pickup; an equalizer including a phasecompensator for equalizing the error signal by the phase compensator togenerate a drive signal; a driver for changing the servo position of theoptical pickup in response to the drive signal; a dropout detector forextracting an envelope signal from the read signal to detect a dropoutof the read signal; and a controller for changing an equalizationcharacteristics of the equalizer in accordance with an amplitude of theenvelope signal during the period of time when the dropout occurs.

[0013] According to the present invention, there is provided a servocontrol apparatus of an optical pickup for reading recorded informationfrom a recording medium to generate a read signal, which comprises anerror signal extractor for extracting an error signal from the readsignal, the error signal indicating a deviation from a servo targetvalue of the optical pickup; an equalizer including a phase compensatorfor equalizing the error signal by the phase compensator to generate adrive signal; a driver for changing the servo position of the opticalpickup in response to the drive signal; a dropout detector forextracting an envelope signal from the read signal to detect a dropoutof the read signal; and a controller for changing an equalizationcharacteristics of the equalizer in response to an occurrence of thedropout.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a sectional view of an optical disc schematicallyshowing the kinds of major defects caused on an optical disc.

[0015]FIG. 2 shows changes of the drive signal supplied to the opticalpickup during a dropout period.

[0016]FIG. 3 shows changes of the focus error signal due to thetransient phenomenon after a dropout period.

[0017]FIG. 4 is a schematic block diagram of the configuration of aservo control apparatus for an optical disc player according to thepresent invention.

[0018]FIG. 5 is a schematic block diagram of the configuration of anequalizer according to a first embodiment of the present invention.

[0019]FIG. 6 is a schematic block diagram of the configuration forgenerating an envelope signal and a dropout detection signal.

[0020] FIGS. 7A-7C are illustrating waveforms when an envelope signal isextracted from an RF read signal and a dropout detection signal isgenerated from the envelope signal.

[0021]FIG. 8 is a flowchart of the procedure for changing thedifferentiation/integration gain characteristics and the phasecompensation characteristics of the equalizer in accordance with theenvelope level.

[0022]FIG. 9 is a graph of gain values against the envelope level whenthe equalizer differentiation/integration gain is changed.

[0023]FIG. 10 is a graph of the envelope level phase margin when theequalizer phase compensation characteristics are changed.

[0024]FIG. 11 is a schematic block diagram of the configuration of theequalizer in a servo control apparatus according to a second embodimentof the present invention.

[0025]FIG. 12 is a diagram of an example in which an error changegeneration process is realized with individual circuit blocks.

[0026]FIG. 13 is a graph of changes in proportional gain and integrationgain against an error change.

[0027]FIG. 14 is a schematic block diagram of the configuration of theequalizer according to a modification of the second embodiment of thepresent invention.

[0028]FIG. 15 is a block diagram of an example of the configuration of aservo control apparatus according to a third embodiment of the presentinvention.

[0029]FIG. 16 is a block diagram of the major parts of a servo controlapparatus, which is realized with circuit blocks, according to a fourthembodiment of the present invention.

[0030]FIG. 17 is a block diagram of the major parts of a servo controlapparatus according to a modification of the fourth embodiment of thepresent invention.

[0031]FIG. 18 a flowchart of the focus-error-hold procedure to beperformed by a controller in a fifth embodiment of the presentinvention.

[0032]FIG. 19 is a flowchart of the equalizer equalizationcharacteristics change procedure in the fifth embodiment of the presentinvention.

[0033]FIG. 20 is a time chart for illustrating an equalizer equalizationchange in the fifth embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0034] The embodiments of the present invention will be described inmore detail with reference to the accompanying drawings. The samereference numerals refer to substantially similar elements throughoutthe drawings.

First Embodiment

[0035]FIG. 4 is a block diagram of the configuration of a servo controlapparatus 10 of an optical disc player according to the presentinvention.

[0036] In FIG. 4, the optical disc 1 is rotated by a spindle motor 8.The spindle motor 8 is provided with an FG signal generator (not shown)for generating one pulse for each predetermined angle of spindlerotation. A generated FG pulse signal is supplied to a controller 23.

[0037] An optical pickup 11 emits a laser light beam on the optical disc1, receives the light beam reflected from the optical disc 1 andgenerates a signal in response to the amount of the received light. Anoptical detector 12 provided in the optical pickup 11 is, for example, a4-partitioned optical detector consisting of four light-receivingelements. Each of the four light-receiving elements receives the lightreflected from a beam spot (not shown), converts it into an electricsignal to output as read signals RB₁-RB₄, respectively. The opticalpickup 11 is further provided with a driver 19 comprising a trackingactuator for making the direction of an objective lens (not shown) orthe reading point thereof, biased in the radial direction, and afocusing actuator for adjusting the focusing point of the beam spot.

[0038] With the above-mentioned configuration, the optical pickup 11reads recorded information from the optical disc 1, and supplies theread signals RB₁-RB₄ to a signal processing circuit 15.

[0039] An RF amplifier is provided in the signal processing circuit 15.The RF amplifier amplifies the read signals RB₁-RB₄, and extracts atracking error signal TE and a focus error signal FE based on theamplified read signals. More specifically, the signal processing circuit15 generates the focus error signal FE using the read signals RB₁-RB₄.The focus error signal FE is a differential signal between two addedsignals obtained by adding read signals from the light-receivingelements diagonally opposing each other. The signal processing circuit15 also generates a tracking error signal TE by the phase differencemethod using the read signals RB₁-RB₄, for example. The tracking errorsignal TE and the focus error signal FE generated in the signalprocessing circuit 15 are supplied to a servo-equalizer (hereinafter,simply referred to as an equalizer) 17.

[0040] As described in detail below, the equalizer 17 is a circuit forcompensating the amplitude and phase of error signals, such as thetracking error signal TE and the focus error signal FE generated in thesignal processing circuit 15. More particularly, the equalizer 17performs equalization processing on error signals so as to havefrequency characteristics suitable for servo control, and supplies theequalized signals to the driver 19 for the optical pickup 11 as drivesignals TD and FD, respectively. The driver 19 generates actuatoroperating signals TDRV, FDRV based on the supplied drive signals TD andFD. The servo control is performed by operating the actuator in theoptical pickup 11.

[0041] Furthermore, according to the present invention, there isprovided a dropout detector 21 for receiving an RF read signal from thesignal processing circuit 15 and for detecting defects formed on theoptical disc 1, i.e., dropouts in the RF read signal; and a controller23 for controlling the equalizer 17 in accordance with the dropoutdetection signal detected by the dropout detector 21, theabove-mentioned FG pulse signal, and signals from the signal processingcircuit 15. The controller 23 is provided with a timer, a memory forstoring various data, such as the servo controlled value, the trackingerror value, or the focus error value, and an operation section forcalculating the coefficient of the equalizer 17 and the like asdescribed below. The controller 23 is configured with, for example, amicrocomputer or a plurality of circuit blocks.

[0042] In the servo control apparatus 10, a control range is determinedin accordance with the transfer function of the actuator and/or thedetection sensitivity of the error detectors including the opticalsystem such that the servo does not get out of place even though adisturbance in the servo system such as wobbling rotation of the discoccurs. Gain characteristics and phase characteristics are set so thatthe system operates with stability within the range. In particular,equalization characteristics are determined so as to secure a sufficientphase margin in the required range of the servo system. Morespecifically, equalization is performed so as to obtain a sufficientphase margin in the frequency range where the gain characteristics of anopen-loop transfer function become 0 dB. Equalization for focus servo,for example, is performed so as to obtain a sufficient phase marginwithin the frequency range of several hundred hertz to severalkilohertz.

[0043]FIG. 5 shows the configuration of the equalizer 17 in theembodiment. The tracking error signal TE and the focus error signal FEextracted in the signal processing circuit 15 are supplied to aproportion/differentiation gain multiplier (APD) 31 to be multiplied bya coefficient bearing a predetermined gain. The multiplied error signalsare supplied to a proportion/differentiation compensator 32 so as toundergo phase compensation.

[0044] The proportion/differentiation compensator 32 is composed of afilter which can shift the phase of an input signal. More specifically,for example, a digital filter of the secondary IIR (infinite impulseresponse filter) and secondary FIR (finite impulse response filter) typecan be used as shown in FIG. 5. The digital filter comprises coefficientmultipliers 33A-33E, delay elements 34A-34D, and adders 35A-35B. Thephase compensation characteristics can be changed by altering each ofthe coefficients A0-A4 of the coefficient multipliers 33A-33E.

[0045] The error signal from the signal processing circuit 15 is alsosupplied to an integration gain multiplier (AI) 37. The integration gainmultiplier (AI) 37 multiplies the supplied error signal by a set gain,and supplies it to an integrator 38. The integrator 38 is so called alow-pass filter (LPF) including a primary IIR filter. In other words,the integrator 38 is an accumulating circuit for the low-pass componentsof error signals, and serves as a holding circuit for holding pastlow-frequency component of the error signals. An output signal from theintegrator 38 is added to an output signal from the above-mentionedproportion/differentiation compensator 32 at an adder 43, and the signalobtained from the adder is supplied to the driver 19 of the opticalpickup 11 as drive signals (TD, FD).

[0046] Thus, when the integration gain of the integration gainmultiplier (AI) 37 increases, the output signal from the integrator 38increases in magnitude, and the component, which is proportional to thecurrent error, decreases in magnitude relatively. As a result, thetracking performance to current disturbances (for example, an unexpecteddisturbance such as dropout of an RF signal due to a defect)deteriorates, making it possible to avoid the influence of thedisturbance temporarily. If this operation is continued, however, thefocused state becomes impossible to be maintained due to a deteriorationof the tracking performance to disturbances (for example, due towobbling rotation of the disc) which should essentially be tracked.Thus, the integration gain and the proportional gain should bedetermined in respect to the above-mentioned tracking performance tocurrent disturbances.

[0047] As is shown in FIG. 5, the gains of theproportion/differentiation gain multiplier (APD) 31 and the integrationgain multiplier (AI) 37 can be changed by a control signal from thecontroller 23. Similarly, the phase characteristics of theproportion/differentiation compensator 32 can be changed by altering acoefficient of the coefficient multiplier (in the case shown in FIG. 5,coefficient A2, a secondary coefficient, of the coefficient multiplier33C) by using a control signal from the controller 23. In theembodiment, the controller 23 generates control signals for theproportion/differentiation gain multiplier (APD) 31, the integrationgain multiplier (AI) 37, and the proportion/differentiation compensator32 based on the amplitude of the envelope signal of the RF read signal.

[0048] The generation of the envelope signal and the dropout detectionsignal will be described below with reference to FIGS. 6, and 7A-7C. Asis shown in FIG. 6, the envelope signal (FIG. 7B) is extracted from anRF read signal (FIG. 7A) by an envelope detector circuit in the signalprocessing circuit 15, and supplied to the controller 23. The envelopesignal is compared with a predetermined discrimination level in acomparator circuit in the dropout detector 21, and a dropout detectionsignal (FIG. 7C) having a predetermined voltage level (Vd) is generated.In the signal processing circuit 15, the generated envelope signal maybe supplied to the controller 23 through a suitable low-pass filter(LPF).

[0049] The operation performed by the controller 23 for controllinggains and phases will be described with reference to the flowchart inFIG. 8 and to FIGS. 9 and 10. It is determined whether or not anydropout is detected in the controller 23(step S1). For example, it isdetermined whether or not a dropout has started by a rise (or fall) inthe envelope signal. The amplitude of the envelope signal is thensampled in response to the dropout detection signal (step S2).

[0050] As is shown in FIG. 9, the controller 23 generates a controlsignal for changing the gains of the proportional differentiation gainmultiplier (APD) 31 and the integration gain multiplier (AI) 37 inaccordance with the sampled amplitude. More particularly, when theenvelope amplitude value (hereinafter referred to as “envelope level”)is below a predetermined value (EL1), a control signal is generated soas to decrease the proportional gain (−6 dB) and increase theintegration gain (+6 dB) because the reliability of the error signal islow. On the other hand, when the envelope level exceeds anotherpredetermined value EL2 (≧EL1), the proportional and integration gainsare not changed. In other words, a control signal indicating the changevalue of 0 dB is generated. When the envelope level is between EL1 andEL2, control is performed so as to change the change value of theproportional gain from −6 dB to 0 dB as the envelope level increases,and at the same time gradually change the integration gain from +6 dB to0 dB.

[0051] As described above, the controller 23 further generates a controlsignal for changing the secondary coefficient (A2) of theproportion/differentiation compensator 32 in accordance with theenvelope level. More particularly, a control signal is generated forchanging the value of the coefficient so as to control transientphenomenon by increasing phase margin when the envelope level is below afirst predetermined value (EL3), as is shown in FIG. 10. On the otherhand, a control signal is generated to indicate that the phase is not tobe changed when the envelope level exceeds a second predetermined valueEL4 (≧EL3). In this manner, a control signal is supplied to theequalizer 17 so as to change the gain and phase in accordance with theenvelope level (step S4).

[0052] It is then determined whether or not the dropout is continuing(step S5). When the dropout is continuing, the procedure goes back tostep S2 to sample the envelope amplitude, and to repeat the stepsdescribed above. When the dropout is terminated (i.e., when the dropoutis disappeared), whether or not the control for changing the gain andphase is to be continued is determined (step S6). The procedure goesback to step S1 to repeat the above steps when the changing control isto be continued. Control exits the process routine when the changingcontrol is not to be continued. Thus, the gain and phase of theequalizer 17 are controlled in accordance with an amplitude of theenvelope signal during the period of time when dropout occurs asdescribed above.

[0053] Although the embodiment has been described taking an examplewhere both the proportional gain and integration gain are changedsimultaneously, only one gain can be changed. In that case, all thatneeds to be done is to change the integration gain relative to theproportional gain. The above-described change value of gains and phasemargin is a mere example. It may be set to a suitable value inaccordance with the servo system to be used.

[0054] As described above, stable high-precision servo control isrealized by changing the gain characteristics and phase characteristicsof the equalizer 17 in accordance with the amplitude of the envelopesignal during the period when dropout occurs.

Second Embodiment

[0055]FIG. 11 is a schematic diagram of the configuration of anequalizer in the servo control apparatus 10 according to a secondembodiment of the present invention. The difference between theembodiment and the first embodiment is that, as is shown in FIG. 11, theequalizer 17 is further provided with a proportion/differentiation gainmultiplier (APD2) 51 and a second integration gain multiplier (AI2) 53so that the gains of the multipliers can be changed by a second controlsignal from the controller 23.

[0056] The controller 23 controls the equalization characteristics ofthe equalizer 17 using a second control signal for changing theintegration gain of the second integration gain multiplier (AI2) 53 orthe proportional gain of the second proportional differentiation gainmultiplier (APD2) 51 in accordance with the change of an error signal inaddition to the control signal for changing gains in accordance with theenvelope level as described in the first embodiment.

[0057] In this embodiment, a change amount or variance of an errorsignal (hereinafter, simply referred to as “error change”) defined belowis used in order to complement gain control by the envelope level.

[0058] Error change=integral value of error (over an angular section ofrotation where a dropout occurs)−integral value of error (over the sameangular section of rotation before the dropout occurs).

[0059] In more detail, an integrated or integral value is obtained foran error signal over a range or section of rotational angle(hereinafter, refereed to as an angular section of rotation) on a trackbefore a dropout occurs. An integral value is, then, obtained over thesame angular section of rotation on a track where a dropout occurs. Theerror change is obtained by subtracting the integral value for theangular section of rotation where a dropout occurs from the integralvalue before the occurrence of the dropout for the same angular sectionof rotation.

[0060] More specifically, for example, the difference between anintegral value of the error signal in one period of the FG signal whendropout occurred and an integral value of the error signal in the sameFG signal period, i.e., in the same angular section on a different trackas that of the former can be used. A drive signal may be used instead ofthe error signal.

[0061] The error change can be generated in the controller 23 asdescribed above. The generation process of the error change can beperformed in circuit blocks shown in FIG. 12. After an error signal isconverted to a digital signal in an analog/digital (A/D) converter 56,an integral value is generated by an integration circuit 57. Theintegral value is supplied to an input terminal on the L terminal of aselector 60 and to a subtracter 59. An output terminal of the selector60 is connected to an FIFO (first-in first-out) memory 58. The outputfrom the FIFO memory 58 is supplied to an input H terminal of theselector 60. A dropout detection signal is supplied to a controlterminal of the selector 60. With such configuration, the selector 60supplies the integral value supplied from the integration circuit 57selectively to the FIFO memory 58 when the dropout detection signal ison the L level, i.e., when dropout is not detected. Or, the selector 60supplies an output signal from the FIFO memory 58 selectively to theFIFO memory 58 when the dropout detection signal is on the H level,i.e., when a dropout is detected.

[0062] The FIFO memory 58 consists of N stages of memories. An FG pulsesignal having N pulses/rotation of the disc is supplied to the FIFOmemory 58 through the gate circuit 60. The content of the memory 58 issequentially supplied to the subtracter 59 in response to the FG pulsesignal. In other words, the output from the subtracter 59 is an integralvalue of the error signal in the current angular section of rotationminus an integral value of the error signal in the same angular sectionof rotation on a different track. The above configuration is designed sothat the supply of an integral value of the error signal to the FIFOmemory 58 is forbidden and data in the FIFO memory 58 is not updatedwhen a dropout is detected. Consequently, the latest integral valuebefore a dropout is detected is stored in the FIFO memory 58.

[0063] The procedure of control operation performed by the controller 23is similar to that in the first embodiment except that, in addition tothe first control signal for changing gains in accordance with theenvelope level, a second control signal for changing gains in accordancewith the error change is supplied to the secondproportion/differentiation gain multiplier (APD2) 51 and the secondintegration gain multiplier (AI2) 53 in the equalizer 17. Moreparticularly, as is shown in FIG. 13, the controller 23 decreases thechange of the proportional gain as the error change decreases when theerror change is below a first predetermined value (EV1) in the negativeregion (in the case shown in FIG. 13, from 0 dB to −0.2 dB). Thecontroller 23 increases the change of integration gain (from 0 dB to 0.4dB) as well as decreases the proportional gain, thus increases theintegration gain. Specifically, it is assumed that a normal error signalcannot be obtained due to the dropout (i.e., the error signal is assumedto be nearly 0). Therefore, control is maintained so as not to track theerror signal affected by the dropout by increasing the integration gainas well as by decreasing the proportional gain. On the other hand, thechange of proportional gain is increased (from 0 dB to 0.4 dB) as theerror change increases when the error change exceeds a secondpredetermined value (EV2) in the positive region. More specifically, thechange of integration gain is decreased (i.e., from 0 dB to −0.4 dB) atthe same time as the proportional gain is increased, thus theintegration gain is decreased. In other words, control is considered tobe greatly deviated from the target state due to disturbances caused bythe rotation of the disc and the like when an error change exceeds thesecond predetermined value (EV2) in the positive region. Thus, theintegration gain is decreased so as to improve the tracking performanceof the servo control apparatus to the disturbances at the same time asthe proportional gain is increased. No compensation of the proportionalgain and the integration gain is performed (i.e., 0 dB) when the errorchange is between the first predetermined value (EV1) and the secondpredetermined value (EV2). The change of the above gains can be suitablyselected in accordance with the servo system or the kind of the disc tobe used.

[0064] As described above, more stable high-precision servo control canbe achieved by adding gain compensation to gain control by the envelopelevel. Although a description has been made of an example where thegains of the second proportion/differentiation gain multiplier (APD2)and the second integration gain multiplier (AI2) are adjusted so as tocompensate the gain of the equalizer 17, the present invention is notlimited to the case but applicable to various cases. For example, thecontrol signal may be determined based on the above error change forchanging the characteristics of the proportion/differentiation gainmultiplier (APD) 31, the integration gain multiplier (AI) 37, and theproportion/differentiation compensator 32 in the equalizer 17 in FIG. 5,which is used for illustrating the first embodiment. Another possibledesign is, as shown in FIG. 14, to change the proportional gain or thephase characteristics of an equalizer having no integration gainmultiplier (AI) 37.

[0065] As mentioned above, the characteristics of the equalizer 17 areadjusted by comparing the error change at the position on the track(i.e., angular section of rotation) where a dropout occurred with thatin the same angular section on the track before the dropout occurred.Accordingly, servo control can be stabilized while a dropout isoccurring even though there is a defect at a certain region over aplurality of tracks to cause the dropout of an RF signal. In addition,the transient phenomenon after the disappearance of the dropout can becontrolled to realize stable high-precision servo control.

Third Embodiment

[0066]FIG. 15 is a block diagram illustrating the configuration of aservo control apparatus 10 according to a third embodiment of thepresent invention. In order to simplify the description, the major partof the configuration of the servo control apparatus 10 is shown incircuit blocks.

[0067] In the embodiment, a focus error signal from a focus errordetector 61 is supplied to the equalizer 17 through a switch 62, and atthe same time, the error value indicated by the focus error signal issupplied to and held in an error value holder 63. The switch 62selectively supplies a focus error signal from the focus error detector61 or the error value from the error value holder 63 to the equalizer 17in response to a logical signal from a logical AND circuit 64. Theswitch 62 takes the AND of a dropout detection signal from a comparator66 and an error change discrimination signal from a discriminator 68 fordiscriminating whether an error change is below a predetermined value.In other words, the logical AND circuit 64 sends out a signal of H levelwhen a dropout is detected and the error change is below a predeterminedvalue, and causes the error value from the holder 63 to be supplied tothe equalizer 17. A control signal from the controller 23 is supplied toa memory 73, and an error value corresponding to the angular section ofrotation where the dropout occurred is output from the memory 73 to theholder 63 as a “hold value”.

[0068] Although a description has been given of an example wherein theerror value held is outputted from the memory 73 only when the errorchange is less than a predetermined value. The held error value may besupplied to the equalizer 17 when a dropout is detected.

[0069] The configuration mentioned above makes it possible to hold theservo control value for an angular section of rotation on a track wherea dropout occurred to the value for the same angular section of rotationon a different track before the dropout occurred so as to realize stablehigh-precision servo control.

Fourth Embodiment

[0070]FIG. 16 is a block diagram illustrating the configuration of aservo control apparatus 10 according to a fourth embodiment of thepresent invention. In the same manner as for the third embodiment, themajor part of the configuration of the servo control apparatus 10 isshown in individual circuit blocks.

[0071] The difference between this embodiment and the above thirdembodiment is that the equalization characteristics of the equalizer 17are changed in response to a dropout detection signal and an errorchange discrimination signal. A logical AND operation of a dropoutdetection signal and an error change discrimination signal are performedin the logic AND circuit 64, and the logical AND signal is supplied tothe switch 62. Specifically, the gain of an amplifier 71 in theequalizer 17 is selectively switched to a low gain value during theperiod when a dropout is detected and the error change is below apredetermined value as shown in FIG. 16.

[0072] A modification of the embodiment is shown in FIG. 17, wherein theequalization characteristics of the equalizer 17 are changed in responseto a dropout detection signal and an error change discrimination signal.The phase compensation value of the proportion/differentiationcompensator 32 in the equalizer 17 is selectively switched to a secondphase compensation value during the period when a dropout is detectedand the error change is less than a predetermined value in theembodiment.

Fifth Embodiment

[0073] Referring now to FIGS. 18-20, a description is made for the servocontrol operation performed by a servo control apparatus 10 according toa fifth embodiment of the present invention. The configuration of theservo control apparatus 10 and the equalizer 17 is similar to that shownin FIGS. 4 and 5.

[0074] In the embodiment, the equalization characteristics of theequalizer 17 including the gain characteristics and phase compensationcharacteristics are changed during and after the occurrence of a dropoutin order to control the transient phenomenon caused in an error signaldue to the occurrence of the dropout. The procedure of the controloperation is described in detail below.

[0075]FIG. 18 is a flowchart of a focus-error-hold operation to beperformed by the controller 23. FIG. 19 is a flowchart of a procedurefor changing the equalization characteristics of the equalizer 17. Theprocedures shown in the flowcharts are performed in parallel with eachother. A time chart for illustrating the change of the equalizationcharacteristics of the equalizer 17 is shown in FIG. 20.

[0076] Referring now to FIGS. 18 and 20, it is determined whether or nota dropout is detected by the controller 23 (step S11). For example, itis determined whether a dropout has started by the rise (or fall) of anenvelope signal. A focus error value FE is then held to a predeterminedvalue (for example, to 0) in response to the dropout detection signal.At the same time, the timer starts time measurement (step S12). Duringthe dropout generation period, a focus error shows a transientoscillation, so the focus-error-hold operation controls the drive signaland stabilizes the operation of the optical pickup.

[0077] It is then determined whether or not the duration of a dropouthas reached a predetermined time (Tdd) (step S13). When the duration ofthe dropout has reached the predetermined time (Tdd), thefocus-error-hold operation is released (step S15). This is to preventservo from becoming unstable because the focus-error hold time beingprolonged more than is required may cause the servo to become unstable.

[0078] In step S13, if the duration has not reached the predeterminedtime (Tdd), it is determined whether or not the -dropout has terminated(step S14). If the dropout has not terminated, the procedure goes backto step S13. On the other hand, if the dropout has terminated, theprocedure goes on to step S15, and the focus-error-hold operation isreleased.

[0079] The presence of a control signal indicating the termination ofservo control is discriminated (step S16) after the focus-error-holdoperation is released in step S15. The procedure goes back to step S11to repeat the above-mentioned procedures when servo control is to becontinued. On the other hand, the current routine exits when servocontrol is to be terminated.

[0080] Referring now to FIGS. 19 and 20, a procedure for changing theequalization characteristics of the equalizer 17 is described. Theequalization characteristics of the equalizer 17 are set to normal gainand phase characteristics, i.e., to the characteristics when no dropoutoccurs (EQ1) (step S21) by the controller 23. It is then determinedwhether or not a dropout is detected (step S22).

[0081] In response to the dropout detection signal, the equalizationcharacteristics are changed to the equalization characteristics to beused in a dropout period (EQ2). At this time, it is preferable toincrease the phase margin, and to change the phase compensationcharacteristics so as to obtain the maximum phase margin. Increasedphase margin makes the convergence of an error signal waveform and alsothe rise of an RF signal faster so as to perform stable servo control.The change of phase compensation characteristics and gains can beperformed by changing the coefficient value of theproportion/differentiation compensator 32 and an integration gain, forexample. The change of a coefficient value may be performed by tableread/write operation in the controller 23.

[0082] It is then determined whether or not the dropout has terminated(step S24). When the termination of the dropout is discriminated, it isdetermined whether or not a predetermined time (Tde) has passed sincethe termination of the dropout (step S25). If the predetermined time(Tde) has passed, a coefficient value is calculated (step S26) forchanging the equalization characteristics of the equalizer 17, and thecharacteristics change is performed based on the calculated coefficientvalue (step S27). It is then determined whether or not the change of theequalization characteristics from EQ2 to EQ1 has been completed by thechange of the coefficient value (step S28). If it has not beencompleted, the procedure goes back to step S26. As is shown in FIG. 20,the characteristics change is gradually performed from EQ2 to EQ1 bycarrying out steps S26-S28 repeatedly.

[0083] When the change of the equalization characteristics is completedin step S28, the presence of a control signal indicating the terminationof servo control is discriminated (step S29). If servo control is to becontinued, the procedure goes back to step S22 to repeat theabove-mentioned procedures. On the other hand, the current routine exitswhen servo control is to be terminated.

[0084] As described above, the convergence of an error signal waveformis made faster and the transient phenomenon after the occurrence of adropout is controlled by changing the equalizer's characteristics inresponse to the occurrence and termination of a dropout or in responseto the time which elapsed since the occurrence and termination of adropout. Thus, stable high-precision servo control is realized.

[0085] Although a description has been given of an example in which thefocus-error hold value is set to 0, a rotation component or a boostcomponent may be used.

[0086] As the various embodiments mentioned above are forexemplification, their application in suitable combinations is possible.Similarly, each of the numerical values shown in the above embodimentsis given only illustrative. The values may be determined properly inaccordance with, for example, the servo system, the kind of disc, or thelike to be used.

[0087] From the above description, it is clear that the presentinvention provides a servo control apparatus which can achieve stablehigh-precision servo control with a high defect treatment performance.

[0088] The invention has been described with reference to the preferredembodiments thereof. It should be understood by those skilled in the artthat a variety of alterations and modifications may be made from theembodiments described above. It is therefore contemplated that theappended claims encompass all such alterations and modifications.

[0089] This application is based on a Japanese Patent ApplicationNo.2000-122467 which is hereby incorporated by reference.

What is claimed is:
 1. A servo control apparatus of an optical pickupfor reading recorded information from a recording medium to generate aread signal, comprising: an error signal extractor for extracting anerror signal from said read signal, said error signal indicating adeviation from a servo target value of said optical pickup; an equalizerincluding a phase compensator for equalizing said error signal by saidphase compensator to generate a drive signal; a driver for changing theservo position of said optical pickup in response to said drive signal;a dropout detector for extracting an envelope signal from said readsignal to detect a dropout of said read signal; and a controller forchanging an equalization characteristics of said equalizer in accordancewith an amplitude of the envelope signal during the period of time whensaid dropout occurs.
 2. A servo control apparatus according to claim 1,wherein said equalizer includes an integrator for integrating said errorsignal to generate an error integration signal and an adder forsupplying an added signal of an error proportional signal which isproportional to said error signal and said error integration signal tosaid driver as said drive signal; wherein, when the amplitude of anenvelope signal during the period of time when said dropout occurs isless than a predetermined value, said controller performs at least oneof decreasing the error proportional signal component in said drivesignal and increasing the error integration signal component to changesaid equalization characteristics.
 3. A servo control apparatusaccording to claim 2, comprising: an integral value calculator forcalculating an integral value of said error signal for eachpredetermined angular section of rotation of a track on said recordingmedium; a memory for storing each of the integral values for onerotation of said recording medium; a variation calculator forcalculating a variation by comparing said integral value for an angularsection of rotation where said dropout occurred with a stored integralvalue for the identical angular section of rotation prior to theoccurrence of the dropout; wherein, when said variation is larger than apredetermined value, said controller performs at least one of decreasingthe error integrated signal component and increasing the errorproportional signal component to change said equalizationcharacteristics.
 4. A servo control apparatus of an optical pickup forreading recorded information from a recording medium to generate a readsignal, comprising: an error signal extractor for extracting an errorsignal from said read signal, said error signal indicating a deviationfrom a servo target value of said optical pickup; an equalizer includinga phase compensator for equalizing said error signal by said phasecompensator to generate a drive signal; a driver for changing the servoposition of said optical pickup in response to said drive signal; adropout detector for extracting an envelope signal from said read signalto detect a dropout of said read signal; and a controller for changingan equalization characteristics of said equalizer in response to anoccurrence of said dropout.
 5. A servo control apparatus according toclaim 4, wherein said controller increases a phase margin in apredetermined frequency range of said equalizer in response to thebeginning of said dropout and decreases said phase margin in response tothe termination of said dropout.
 6. A servo control apparatus accordingto claim 4, wherein said controller includes a timer for measuring theelapsed time from the termination of said dropout and performs controlfor changing said equalization characteristics of said equalizer basedon the output of said timer.
 7. A servo control apparatus according toclaim 6, wherein said controller changes said equalizationcharacteristics of said equalizer gradually in accordance with theelapsed time from the termination of said dropout.
 8. A servo controlapparatus according to claim 4, wherein said equalizer includes anintegrator for integrating said error signal to generate an errorintegration signal and an adder for supplying an added signal of anerror proportional signal which is proportional to said error signal andsaid error integration signal to said driver as said drive signal;wherein said controller performs, during the occurrence of the dropout,at least one of decreasing the error proportional signal component insaid drive signal and increasing the error integration signal componentto change said equalization characteristics.